1. Field of the Invention
The present invention relates to a method for preparing a water-absorbent polymer composite and an accumulated material thereof. The accumulated material of the water-absorbent composite prepared by the invention is thin and pliable and can be opened. The water-absorbent polymer composite and its accumulated material prepared by the invention are favorable for producing sanitary goods such as paper diapers, sanitary napkins, and other water-absorbent articles such as industrial materials.
2. Description of the Related Art
Heretofore water-absorbent polymers capable of absorbing a large amount of water have been widely used for sanitary goods, industrial materials, etc. In case where a water-absorbent polymer is combined with any other material to form its composite as in paper diapers, it is desired that the polymer is well fixed to the material before and after water absorption, the composite is thin and pliable, and the polymer content of the composite is high.
JP-A 63-63723 discloses a composite that comprises a hydrophilic substrate with fibers at least partially wrapped therein, and this is produced by kneading and dispersing hydrophilic fibers with a water-absorbent polymer wetted and swollen with water or a water-containing solvent followed by drying and grinding the resulting dispersion, or by polymerizing a water-soluble ethylenic unsaturated monomer while mixed with hydrophilic fibers followed by drying and grinding the resulting polymer mixture. The process disclosed gives a composite that comprises a water-absorbent polymer and fibers. However, the composite obtained according to the process must be ground before use. This inevitably gives broken or pulverized fiber pieces, and is therefore problematic in that fiber debris and fine water-absorbent polymer particles are formed and the non-fixed water-absorbent polymer is released. Another problem is that the molecular chains of the water-absorbent polymer are cut by mechanical shock during the kneading and dispersing operation and the water-absorbing capacity of the composite is therefore inevitably lowered. Still another problem is that, while the components are kneaded and dispersed, air is led into the resulting dispersion to form voids inside the water-absorbent polymer, and, as a result, the reduction in the water-absorbing capacity under pressure of the composite and the reduction in the bulk density thereof are also inevitable. In addition, in the composite produced according to the process, the fibers are always at least partially wrapped in the water-absorbent polymer, or that is, the process could not give a composite comprising fibers partially adhered to the surface of a water-absorbent polymer as in the present invention.
JP-B 5-58030 discloses a water-absorbent article which comprises a fibrous substrate at least partially composed of hydrophobic fibers and a water-absorbent polymer adhering to the substrate. The water-absorbent article is characterized in that at least a part of the water-absorbent polymer is nearly spherical to wrap the substrate fiber and discontinuously adheres to the substrate. Since the substrate is fibrous, the technique ensures the pliability of the composite. In addition, the water-absorbent polymer is fixed to the substrate. However, since the water-absorbent polymer wraps the fibers therein, it is inevitable that the fibers interfere with the swelling of the water-absorbent polymer. In addition, since the water-absorbent polymer is discontinuously adhered to the fibers, the ratio of water-absorbent polymer/fibers must be small. Naturally, even though a water-absorbent polymer discontinuously adheres to a fiber, the fiber may interfere with the polymer swelling when the polymer-to-polymer distance is narrow. In view of this, the ratio of water-absorbent polymer/fibers could not be enlarged. Still another problem with the technique disclosed is that the substrate usable therein is limited to hydrophobic fibers for the morphology control of the water-absorbent polymer.
JP-A 11-93073 discloses a polymer-fiber composite in which a nearly spherical water-absorbent polymer is discontinuously fixed on the surfaces of nonfabricated fibers and the non-fabricated fibers are accumulated, or the nonfabricated fibers bond to each other via the water-absorbent polymer. Naturally from the point of view that the water-absorbent polymer adheres to the fibers, it may be said that the fixation of the water-absorbent polymer is realized. However, since the water-absorbent polymer adheres to the surfaces of the fibers, the adhesion morphology is inevitably limited to point adhesion or line adhesion, and the adhesion strength in dry will be unsatisfactory. This causes a problem in that the fixation retention in dry is unsatisfactory. Still another problem is that, when the water-absorbent polymer has absorbed water and its surface is swollen and elongated, it readily peels off and moves away.
The patent publication describes an embodiment of fibers wrapped in a water-absorbent polymer. However, like in the above-mentioned JP-B 5-58030, this is defective in that the swelling retardation of the water-absorbent polymer by the fibers is inevitable. Moreover, since the fibers bonds to each other via the water-absorbent polymer, the composite is difficult to be opened. If it is forcedly opened, the water-absorbent polymer itself therein will be damaged, therefore causing a problem in that the water absorbency of the composite will lower and a non-fixed water-absorbent polymer will be released. For these reasons, it is difficult to open the composite and uniformly mix it with any other material.